The components of high-temperature mechanical systems, such as, for example, gas-turbine engines, must operate in severe environments. For example, the high-pressure turbine blades and vanes exposed to hot gases in commercial aeronautical engines typically experience metal surface temperatures of about 900-1000° C., with short-term peaks as high as 1150° C. A portion of a typical metallic article 10 used in a high-temperature mechanical system is shown in FIG. 1. The blade 10 includes a Ni or Co-based superalloy substrate 12 coated with a thermal barrier coating (TBC) 14. The thermal barrier coating 14 includes a thermally insulative ceramic topcoat 20 and an underlying metallic bond coat 16. The topcoat 20, usually applied either by air plasma spraying or electron beam physical vapor deposition, is currently most often a layer of yttria-stabilized zirconia (YSZ) with a thickness of about 300-600 μm. The properties of YSZ include low thermal conductivity, high oxygen permeability, and a relatively high coefficient of thermal expansion (CTE). The YSZ topcoat 20 is also made “strain tolerant” by depositing a structure that contains numerous pores and/or pathways. The consequently high oxygen permeability of the YSZ topcoat 20 imposes the constraint that the metallic bond coat 16 must be resistant to oxidation attack. The bond coat 16 is therefore sufficiently rich in Al to form a layer 18 of a protective thermally grown oxide (TGO) scale of Al2O3. In addition to imparting oxidation resistance, the TGO bonds the ceramic topcoat 20 to the substrate 12 and bond coat 16.
The adhesion and mechanical integrity of the TGO scale layer 18 is very dependent on the composition and structure of the bond coat 16. Ideally, when exposed to high temperatures, the bond coat 16 should oxidize to form a slow-growing, non-porous TGO scale that adheres well to the superalloy substrate 12. Conventional bond coats 16 are typically either (i) an MCrAlY overlay (where M=Ni, Co, NiCo, or Fe) having a β-NiAl+γ-Ni phase constitution or (ii) a platinum-modified diffusion aluminide having a β-NiAl phase constitution. The Al content in either of these types of coatings is sufficiently high that the Al2O3 scale layer 18 can “re-heal” following repeated spalling during service of the turbine component.
However, as a result of this Al enriched composition and the predominance of the β-NiAl in the coating microstructure, these coatings are not compatible with the phase constitution of the Ni-based superalloy substrates, which have a gamma-Ni phase and a gamma prime-Ni3Al (referred to herein as γ-Ni+γ′-Ni3Al or γ+γ′) microstructure. When applied to a superalloy substrate having a γ-Ni+γ′-Ni3Al microstructure, Al diffuses from the coating layer to the substrate. This Al interdiffusion depletes Al in the coating layer, which reduces the ability of the coating to sustain Al2O3 scale growth. Additional diffusion also introduces unwanted phase changes and elements that can promote oxide scale spallation. A further drawback of β-NiAl-based coatings is incompatibility with the γ-Ni+γ′-Ni3Al-based substrate due to CTE differences.
Another approach to depositing a protective coating on a γ-Ni+γ′-Ni3Al-based metallic article 28, described in U.S. Pat. Nos. 5,667,663 and 5,981,091 to Rickerby et al., is shown in FIG. 2A. A superalloy substrate 30 is coated on an outer surface with a layer 32 of Pt and then heat-treated. Referring to FIG. 2B, during this heat treatment, interdiffusion occurs, which includes the diffusion of Al from the superalloy substrate 30 into the Pt layer 32 to form an Al-enriched Pt-modified outer surface region 34 on the superalloy substrate (FIG. 2B). An Al2O3 TGO scale layer 38 may then form on the surface-modified region 34 and a ceramic layer topcoat 40 may also be deposited using conventional techniques. However, since transition metals from the superalloy substrate 30 are also present in the surface modified region 34, it is difficult to precisely control the composition and phase constitution of the surface region 34 to provide optimum properties to improve adhesion of the TGO scale layer 38. Rickerby et al. further suggest that this platinizing and heat treatment process may include the incorporation up to 0.8 wt % of Hf or Y into the platinum-enriched surface layer, but no specific deposition methods or pack compositions were provided to achieve this surface layer composition.
Copending application U.S. Ser. No. 10/439,649, incorporated herein by reference, describes alloy compositions suitable for bond coat applications. The alloys include a Pt-group metal, Ni and Al in relative concentration to provide a γ+γ′ phase constitution, with γ referring to the solid-solution Ni phase and γ′ referring to the solid-solution Ni3Al phase. In these alloys, a Pt-group metal, Ni and Al, are present, and the concentration of Al is limited with respect to the concentrations of Ni and the Pt-group metal such that the alloy includes substantially no β-NiAl phase. These alloys are shown in the region A in FIG. 3.
Preferably, the ternary Ni-Al-Pt alloy in the copending '649 application includes less than about 23 at % Al, about 10 at % to about 30 at % of a Pt-group metal, preferably Pt, and the remainder Ni. Additional reactive elements such as Hf, Y, La, Ce and Zr, or combinations thereof, may optionally be added to or present in the ternary Pt-group metal modified γ-Ni+γ′-Ni3Al alloy and/or improve its properties. The addition of such reactive elements tends to stabilize the γ′ phase. Therefore, if sufficient reactive metal is added to the composition, the resulting phase constitution may be predominately γ′ or solely γ′. The Pt-group metal modified γ-Ni+γ′-Ni3Al alloy exhibits excellent solubility for reactive elements compared to conventional β-NiAl-based alloys, and in the '649 application the reactive elements may be added to the γ+γ′ alloy at a concentration of up to about 2 at % (˜4 wt %). A preferred reactive element is Hf. In addition, other typical superalloy substrate constituents such as, for example, Cr, Co, Mo, Ta, and Re, and combinations thereof, may optionally be added to or present in the Pt-group metal modified γ-Ni+γ′-Ni3Al alloy in any concentration to the extent that a γ+γ′ phase constitution predominates.
The Pt-group metal modified alloys have a γ-Ni+γ′-Ni3Al phase constitution that is both chemically, physically and mechanically compatible with the γ+γ′ microstructure of a typical Ni-based superalloy substrate. Protective coatings formulated from these alloys will have coefficients of thermal expansion (CTE) that are more compatible with the CTEs of Ni-based superalloys than the CTEs of β-NiAl-based coatings. The former provides enhanced coating stability during the repeated and severe thermal cycles experienced by mechanical components in high-temperature mechanical systems.
When thermally oxidized, the Pt-group metal modified γ-Ni+γ′-Ni3Al alloy coatings grow an α-Al2O3 scale layer at a rate comparable to or slower than the thermally grown scale layers produced by conventional β-NiAl-Pt bond coat systems, and this provides excellent oxidation resistance for γ-Ni+γ′-Ni3Al alloy compositions. When the Pt-metal modified γ+γ′ alloys further modified with a reactive element such as, for example, Hf, and applied on a superalloy substrate as a coating, the growth of the TGO scale layer is even slower than comparable coating compositions without Hf addition. After prolonged thermal exposure, the TGO scale layer further appears more planar and has enhanced adhesion on the coating layer compared to scale layers formed from conventional β-NiAl-Pt coatings.
In addition, the thermodynamic activity of Al in the Pt-group metal modified K-Ni+K′-Ni3Al alloys can, with sufficient Pt content, decrease to a level below that of the Al in Ni-based superalloy substrates. When such a Pt-group metal modified γ-Ni+γ′-Ni3Al alloy coating is applied on a superalloy substrate, this variation in thermodynamic activity causes Al to diffuse up its concentration gradient from the superalloy substrate into the coating. Such “uphill diffusion” reduces and/or substantially eliminates Al depletion from the coating. This reduces spallation in the scale layer, increases the long-term stability of the coating and scale layers, and would greatly enhance the reliability and durability of a thermal barrier coating system.
The Pt-group metal modified K-Ni+K′-Ni3Al alloy may be applied to a superalloy substrate using any known process, including for example, plasma spraying, chemical vapor deposition (CVD), physical vapor deposition (PVD) and sputtering to create a coating and form a temperature-resistant article. Typically this deposition step is performed under non- or minimal oxidizing conditions.
As described earlier, when the Pt-group metal modified γ+γ′ alloys described in the '649 application are formulated with other reactive elements such as, for example, Hf, and applied on a superalloy substrate as a coating, the growth of the TGO scale layer is even slower than comparable coating compositions without Hf addition. After prolonged thermal exposure, the TGO scale layer further appears more planar and has enhanced adhesion on the coating layer compared to scale layers formed from conventional β-NiAl-Pt bond coat materials. As such, inclusion of a reactive element in the Pt-metal modified γ+γ′ alloys described in the '649 application is highly desirable.